Quick answer: a flower beetle passes through four stages: egg, grub, pupa and adult. The egg hatches in the substrate within a few weeks, the grub grows through three stages (L1, L2 and L3) while eating the bed it lives in, and the fully grown grub builds a hard cocoon from the bed and its own droppings, pupates inside and digs out as a beetle several weeks later. Keepers put the whole cycle at roughly three to six months for the sun beetle, while the Sapphire flower beetle takes about a year with a winter rest.
I rear several kinds of flower beetle from grubs, my sun beetles among them, and the grub at the top of this page is one of my Protaetia formosana. The kinds kept as pets, such as the sun beetle (Pachnoda marginata), the African fruit beetle (Pachnoda fissipunctum), Derby's flower beetle (Dicronorhina derbyana) and the Sapphire flower beetle (Protaetia speciosa cyanochlora), are all flower chafers, the subfamily Cetoniinae of the scarab family, and they run through the same stages. For the enclosure and the care of adults, see my flower beetle care guide.
Flower Beetle Eggs
A mated female digs into the bed to lay, and in a laboratory study of the sorghum chafer, Pachnoda interrupta, the females laid their eggs singly. Keepers give two to four weeks for sun beetle eggs to hatch, sooner in warmth; the sorghum chafer's hatched in 6 to 16 days at 28°C.
I have found no reliable published egg count for the sun beetle. The sorghum chafer laid about 1.75 eggs a day for more than two months. One keeper moves any eggs found in a bed change to a separate tub, away from the adults.
Flower Beetle Larvae: L1, L2 and L3
The grub grows through three larval stages, or instars, shedding its skin between each, and sellers list them as L1, L2 and L3. The textbook way to tell them apart is head width: the hard head capsule stays the same width between moults, as the University of Minnesota's guide to white grubs explains, and the Pachnoda interrupta study separated its three stages that way. Which stage to buy, tub sizes and bed changes are covered in my guide to keeping beetle larvae.
Reptile keepers may know these grubs already: sun beetle larvae are sold in the UK as livefood, as fruit beetle grubs or Pachnoda grubs.
Below are final-stage (L3) grubs of three of the kinds I rear: Protaetia niveoguttata, Caelorrhina kolbei and Protaetia culta. Look for the C shape, the bristles and the row of small rust-red breathing holes (spiracles) along each side. The hard brown head capsule is clearest on the Caelorrhina grub, at its lower end. Where the rear end looks grey, that is gut contents showing through the skin, as the same Minnesota guide notes for white grubs.
Why the grubs crawl on their backs
Put a flower beetle grub on a flat surface and it rolls over and travels upside down. The Australian Faunal Directory describes the subfamily's grubs moving on their backs using folds densely clothed with strong bristles, and WSL, the Swiss forest research institute, notes the same of the rose chafer, which is also native to Britain: its grubs crawl on their backs, where cockchafer grubs lie on their sides.
What Flower Beetle Grubs Eat
The grub eats the bed it lives in, so the substrate is its whole diet. Wild flower chafer larvae live in decaying vegetable matter, rotten wood and soil, and laboratory work on the sun beetle describes its grubs as humus feeders. In a tub, that means fermented flake soil with crushed oak and beech leaf litter and crumbling rotten hardwood mixed through, damp enough to clump when squeezed but never dripping. Leave out pine and other softwoods, and skip coir: it holds moisture but isn't food.
Kinshi, the fungus-run hardwood that stag beetle keepers use, is optional for flower beetle grubs. As the grubs eat, the bed sinks and darkens with their droppings, so top it up with fresh leaf litter and rotten wood. Air any new flake soil for a few days and check it is not warm before grubs go in, because a bed that is still fermenting can heat itself.
Grubs Live Together, but Crowding Costs Size
Stag beetle grubs can injure each other and are reared one to a tub, but flower beetle grubs share a bed: breeders give 20 to 30 grubs in 10 to 15 litres, or 15 in 5 litres. Keepers report that crowded grubs grow into smaller adults, and a grub that runs short of food pupates early and small.
The Final Grub: Signs It Is About to Pupate
Months after hatching, a fully grown L3 that is ready to pupate slows down and shortens, and its feeding changes: a study of sun beetle grubs found that the grub empties its gut and then refills it before pupating, which the researchers suggest may be linked to building the pupal chamber from the gut contents. It then works the bed into an oval cell around itself, and if it builds against the side of the tub you may be able to see it.
The Flower Beetle Cocoon
The cocoon is made from the bed itself: substrate packed into an oval shell, held together with the grub's own secretions and lined with its droppings.
Here is a Chlorocala africana cocoon of mine beside a grub of the same kind. From the outside it looks like a lump of the bed.
Keepers give three to eight weeks in the cocoon for the sun beetle, a UK zoo gives about five weeks as a pupa, and most figures for other flower beetles fall between one and two months from a finished cocoon to a beetle on the surface. In the wild, the Sapphire's sister species finishes its change in autumn and sits out the winter in the cell, and Britain's rose chafer likewise pupates in autumn but does not appear until spring.
From the moment a cocoon appears, leave the tub alone. The pupa inside cannot repair a broken cocoon, and handling a pupa, as one keeper puts it, does much more harm than good.
If a cocoon breaks
- A grub that can still crawl. Settle it on a small tub of the same damp bed and leave it. One keeper of Derby's flower beetle had a grub dig out of a broken cell and build a new one.
- A pupa (pale and soft, legs folded against the body, able only to flex its abdomen). It cannot build a new cell, so keepers make one: a smooth oval hollow a little larger than the pupa, pressed into damp, fine substrate or cut into florist's foam soaked in water first. Lay the pupa in it, keep the lid on so the air stays humid, and leave it alone.
- A beetle. If the adult is already formed and coloured, set it on the bed where it can dig in.
Emergence: The New Beetle Waits in Its Cell
Inside the cocoon, the pupa sheds its last skin and a soft, pale beetle comes out, but it does not dig straight up. It waits in the cell while its shell hardens and its colours come through, up to about two weeks by one Belgian breeder's account, and WSL gives rose chafers several weeks to harden and colour. It digs out when it is ready, so a quiet cocoon is no reason to dig.
Do not force a fresh beetle to feed, and do not expect eggs straight away: in a laboratory study of Protaetia brevitarsis, a relative of the Sapphire, at 25°C, 73 per cent of eggs were laid between three and eight weeks after the females emerged.
The Adult Beetle
On the surface, the adult feeds, mates and lays into the same bed, and the cycle starts again. Adult flower beetles have weak mouthparts suited to soft food, so they live on soft ripe fruit and beetle jelly, which stays fresh in the tub for longer than fruit does. They fly, so the lid matters more than the walls, and the African fruit beetle care guide covers adult care for a typical Pachnoda.
Most adults live a few months. Sun beetles are often quoted at around five months, though in a study at a UK college zoo some lived for more than ten. Adults and grubs that are ready to go are listed on the beetles for sale page.
How Long the Whole Cycle Takes
- Sun beetle: roughly three to six months from egg to adult by keepers' figures, quicker at the warm end. The African fruit beetle is kept the same way.
- Derby's flower beetle: breeders' figures run from eight or nine months to about a year.
- Sapphire flower beetle: about a year with a winter rest, and one German breeder gives eleven to thirteen months. Whether it needs the cold rest at all is argued over, and the Sapphire flower beetle care guide sets out both sides.
A cold room stretches all of these figures.
What Goes Wrong
Grubs on the surface. Waterlogged soil holds less oxygen, which is why heavy rain drives grubs to the surface outdoors. Keepers add sealed tubs with poor airflow and beds that are mostly droppings, and a new bed that is still fermenting can warm itself. Check that the bed clumps without dripping, is not warm or sour and still has food in it, and replace it if not.
Small adults. Crowding, or a bed that ran short of food.
Eggs thrown out with old bedding. Spent substrate can hold eggs and tiny grubs, so sift it by hand and never tip it onto a compost heap: in England and Wales, releasing or allowing to escape any animal of a kind not ordinarily resident in and not a regular visitor to Great Britain in a wild state is an offence under section 14 of the Wildlife and Countryside Act 1981, and Scotland has its own version of the rule.
Frequently Asked Questions
How long do flower beetle larvae take to pupate?
It depends on the species and the warmth. A UK zoo gives three to five months as a grub for the sun beetle, a related Pachnoda finished its grub stage in 41 to 71 days at 28°C in a laboratory, and one keeper puts Derby's flower beetle at around seven to eight months.
How long are flower beetles in the cocoon?
Keepers give three to eight weeks for the sun beetle, and most figures for other flower beetles fall between one and two months.
Should I open a flower beetle cocoon to check on it?
No. The pupa inside cannot repair a broken cocoon, so leave it until the beetle digs itself out. If one breaks by accident, give the pupa an artificial cell.
Why are my fruit beetle grubs coming to the surface?
Usually because of the bed: too wet and short of air, still fermenting and warm, or used up and full of droppings. The fix is a fresh bed.
What do fruit beetle grubs eat?
The bed they live in: flake soil with rotted hardwood leaves and crumbling rotten wood mixed through. The bed is the whole diet, and coir holds moisture but isn't food.
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